The present invention relates to a method for applying a tread to a tyre casing, and then particularly for remoulding or retreading vehicle tyre casings. The invention also relates to a wire element for use when applying a tread to a tyre casing.
Worn tyres are retreaded by removing the worn tread so that only the old tyre casing remains, whereafter a new tread is applied to the casing. The tyre is retreaded by first applying a layer of binder, normally a non-vulcanised rubber binder, on the casing surface that faces towards the tread, whereafter the new prevulcanised tread is applied and the rubber binder heated, whereafter pressure is applied to the tread so as to press the tread firmly against the tyre casing.
In order for the retread to function properly, it is necessary to ensure that the tread is seated firmly on the tyre casing, and correct heating of the rubber and correct application of an external pressure onto the tread are required in order to obtain a successful result.
One method of heating the rubber binder used in recent times is to bake metallic filaments in the rubber and to deliver an electric current to the filaments so as to heat the rubber. The tyre is normally fitted with a so-called envelope that can be applied externally around the tread so as to extend down over the sides of the tyre casing, said envelope then being sealed at the bead part by a so-called open rim. The envelope is connected to a subpressure source and therewith pressed against the tread and the tyre casing. The whole of the tyre assembly is then placed in a pressure chamber in which an overpressure is generated to hold the envelope pressed against the tyre casing. Heating of the rubber binder and pressing of the envelope against the tread and the tyre casing results in vulcanisation of the binder, which is also firmly vulcanised to the tyre casing and the tread at the same time. The method also involves evacuating the vulcanisation gases that occur and also the air that is pressed out from the region between the tyre casing and the tread, by connecting the tyre assembly to a source of subpressure.
The problems that still remain with respect to the tread loosening from the tyre casing are not due primarily to a rubber binder of poor quality, but to the formation of air inclusions and vulcanisation gas inclusions as the envelope is applied to the tread, said air and gases thus being trapped by virtue of the outer edge of the tread being pressed firmly against the tyre casing so as to prevent them from being evacuated to the edge of the tyre casing/the tread and therewith evacuated by connection to the subpressure source.
Accordingly, one object of the present invention is to provide a method with which the aforesaid problems can be avoided, and particularly which will enable air and vulcanisation gas inclusions to be evacuated from the binding rubber layer.
This object is achieved by the invention in that the wires used for heating the layer of rubber binder are multiwire elements which extend out to at least the edge of the tread.
By constructing and arranging the wires in the manner proposed in accordance with the invention there is obtained between the wires a given small space that extends to the edge of the tread and that allows enclosed air and vulcanisation gases to pass through and out to the space externally of the tread, from where the air and gases can be removed by suction.
Another object of the invention is to provide a wire element which will enable air and vulcanisation gases to be transported along said wire element to the edge of the tread.
In accordance with the invention this object is achieved by heating the rubber binding layer with the aid of a multiwire element.